1. Field of the Invention
The present invention relates to a method of producing a sample of a semiconductor substrate which is intentionally applied with quantified low-concentration metal or organic matter contamination. The sample is used for evaluating low-concentration contamination generated during semiconductor producing processes and device characteristics or evaluating a washing effect for low-concentration contamination.
2. Description of the Prior Art
Following advancement of fine structures of very large scale integrated circuits, the demands have been even more increased for the cleaning of the surfaces of semiconductor substrates. In particular, metal impurities cause a serious problem when adhered to the surfaces of the semiconductor substrates since the electric characteristics of semiconductor devices are largely degraded. In general, it has been reported that the surface metal contamination concentration of the semiconductor substrate should be no greater than 1.times.10.sup.10 atoms/cm.sup.2. It has also been reported that organic matter in the cleanroom environment deteriorates the electric characteristics of the semiconductor devices. Since thinner gate oxide films have been developed, reduction of the metal and organic impurities is required more than before.
Under these circumstances, it is becoming more and more important to produce semiconductor substrates contaminated with impurities and perform electric characteristic evaluation and washing evaluation for the semiconductor devices.
There have been three types of conventional techniques for producing contaminated samples or standard samples with respect to metal impurities.
In Japanese First (unexamined) Patent Publication No. 6-249764, a silicon wafer or substrate is dipped into an alkaline hydrogen peroxide solution containing metal impurities of constant concentration and then rinsed with pure water. According to this method, contaminated samples can be provided wherein in-plane distribution, depth-direction distribution and the total amount of the metal impurities, such as iron or nickel, are controlled to be constant.
In Japanese First (unexamined) Patent Publication No. 1-243536, a suitable method for contamination is determined by examining the formation enthalpy of contaminating metallic element oxide. Since the oxide formation enthalpy of aluminum, iron or the like is large, the metal of desired concentration is dropped into purified water or an oxidizing treatment liquid, and then a semiconductor substrate is dipped thereinto and contaminated with the metal. On the other hand, in case of metal, such as copper, having a small oxide formation enthalpy, a semiconductor substrate is contaminated in a contaminating treatment liquid containing copper after removing a native oxide film. In this manner, contaminated samples of desired metals can be selectively provided.
In Japanese First (unexamined) Patent Publication No. 7-169810, a spin-coat method is used which differs from the foregoing two chemical solution dipping methods. Several milliliters of a contaminating liquid diluted to constant concentration are dropped onto a silicon wafer with a native oxide film formed through washing. The wafer is rotated at low speed so as to diffuse droplets of the contaminating liquid all over the hydrophilic wafer. After standing for a given time, the wafer is rotated at high speed to be dried. According to this method, dispersion among the wafers is small, and the metal impurity concentrations on the silicon wafers are increased in proportion to increase of the metal impurity concentrations in the solutions with respect to any metals. In this manner, metal-contaminated samples of the desired concentrations can be provided.
However, the foregoing three conventional techniques have the following problems:
In Japanese First Patent Publication No. 6-249764, since the alkaline hydrogen peroxide solution is used, while quantified contamination can be achieved with respect to the metal (iron, nickel or the like) which is liable to produce oxides or whose solubility product is large on the alkaline side, quantified contamination is difficult with respect to the metal, such as copper. Further, due to use of a strong alkali, the silicon surface is etched so that the surface roughness thereof is increased.
On the other hand, in Japanese First Patent Publication No. 1-243536, for avoiding the foregoing problem, the contamination by copper is performed after removal of the native oxide film in consideration of the oxide formation enthalpy of the metal impurities.
In the foregoing two publications, although the quantified contamination can be applied to a plurality of the wafers at one time by use of batch processing, controlling the amount of concentration and achieving low-concentration contamination are very difficult. In particular, when using ammonia water or the like, since iron or aluminum is liable to be entrained from the atmosphere due to its property, they could exist in the solution at an amount no more than 1 ppb. Then it is possible that the contamination level of 1.times.10.sup.10 to 1.times.10.sup.13 atoms/cm.sup.2 occurs without adding the metal impurities. Since the control of the metal concentration in the solution in the range of no more than 1 ppb is difficult, the contamination level of no more than 1.times.10.sup.11 atoms/cm.sup.2 cannot be achieved on a practical basis.
In the spin-coat method of Japanese First Patent Publication No. 7-169810, since the wafers are contaminated one by one, it takes time for the forced contamination. Since the contamination can be achieved only to the hydrophilic surface, the final washing before the spin-coating must be carried out so as to form the natural oxide film. Since the contaminating liquid is dropped at the center of the wafer which is then rotated, in-plane dispersion is large and segregation tends to occur at the center. Since some metals tend to be entrapped into the native oxide films while some metals tend to segregate at the silicon interface, the actual contamination states are not reflected due to the fact that the contamination is applied on the all native oxide film surface.
According to the foregoing three types of the conventional techniques, only contamination levels of no less than 1.times.10.sup.12 atoms/cm.sup.2 can be achieved. In Japanese First Patent Publication No. 6-249764, it is described that a contamination level of 1.times.10.sup.10 atoms/cm.sup.2 can be achieved. However, as described before, purity control is difficult when using the strong alkaline ammonia water so that a contamination level of 1.times.10.sup.10 to 1.times.10.sup.13 atoms/cm.sup.2 may occur. Thus, it has been difficult to achieve quantified contamination with good reproducibility.
Specifically, in the foregoing three types of the conventional techniques, there has been a problem that the low-concentration metal contamination cannot be achieved at gradual or stepwise concentrations and with required reproducibility. The main cause of this resides in controlling the surface contamination concentration by controlling the metal concentration in the contaminating liquid. Further, no method has been presented for quantified contamination by organic matters.